There are many electronic intrusion detection systems which employ a fiber optic cable as a sensor. Typically, mechanical attenuators respond to an external force on the fiber optic cable to produce a microbend in the fiber optic cable. A light signal passing through the fiber optic cable from a first cable end responds to the microbend in two ways.
In the first fashion, the microbend causes the light intensity to diminish at a second cable end which is readily detected by a light detector. Thus, when the light intensity received at the second cable falls below a predetermined threshold level an alarm signal is generated.
The second way that the light signal is affected by the microbend is that there is an increase in back scattered light. Using an optical time domain reflectometer, the location of the microbend along the cable length is readily determined.
There are several problems with the electronic intrusion detection systems of the prior art. First, of those devices in the prior art, all are designed as permanent installations about a fence or other permanent type structure. Thus, the attenuators are expensive to make and expensive to install. Further, once they are installed to the fence, they are not suitable for quick removal and relocation.
One example of this type of attenuator is described in U.S. patent application Ser. No. 10/141,402 which teaches a spring loaded mechanical attenuator which is installed at various points along a fence. These complicated attenuators must be installed and maintained by competent installation and maintenance personnel. Further, when these attenuators are tripped they must be reset which is a time consuming process and requires knowledge of the inner workings of the attenuators.